1. Field of the Invention
This invention relates to flow field plate constructions for bipolar plates comprising an internal coolant flow field for use in fuel cell stacks subject to freezing temperatures in operation or storage. In particular, it relates to design features in the vicinity of the reactant inlet and outlet ports of the plates for accommodating the formation of ice.
2. Description of the Related Art
Solid polymer electrolyte or proton exchange membrane fuel cells (PEMFCs) electrochemically convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. PEMFCs generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). In a typical fuel cell, an anode flow field plate and a cathode flow field plate, each comprising numerous fluid distribution channels for the reactants, are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1 V, a plurality of cells is usually stacked together in series for commercial applications. In such a stack, the anode flow field plate of one cell is thus adjacent to the cathode flow field plate of the adjacent cell. For assembly purposes, a set of anode flow field plates is often bonded to a corresponding set of cathode flow field plates prior to assembling the stack. A bonded pair of an anode and cathode flow field plates is known as a bipolar plate assembly. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
Along with water, heat is a significant by-product from the electrochemical reactions taking place within the fuel cell. Means for cooling a fuel cell stack is thus generally required. Stacks designed to achieve high power density (e.g. automotive stacks) typically circulate liquid coolant throughout the stack in order to remove heat quickly and efficiently. To accomplish this, coolant flow fields comprising numerous coolant channels are also typically incorporated in the flow field plates of the cells in the stacks. The coolant flow fields are typically formed on the electrochemically inactive surfaces of both the anode side and cathode side flow field plates (or fuel flow field plate and oxidant flow field plate respectively). By appropriate design, a sealed coolant flow field is created when both anode and cathode side plates are mated together into a bipolar plate assembly. The sealed coolant flow field can thus serve to distribute coolant evenly throughout the cells while keeping the coolant reliably separated from the reactants.
To provide both reactants and the coolant to and from the individual cells in the stack, a series of ports are generally provided at opposing ends of the individual cells such that when the cells are stacked together they form manifolds for these fluids. Further required design features then are passageways in the plates to distribute the bulk fluids in these formed manifolds to and from the various channels in the reactant and coolant flow fields in the plates. Herein, these passageway regions are referred to as the transition regions. The transition regions can themselves comprise numerous fluid distribution channels, e.g. oxidant and/or fuel transition channels.
Another desirable feature in the flow field plates can include the use of what are known in the art as backfeed ports. Such ports allow for bulk fluids to initially be distributed from the formed manifolds to the “back” or inactive sides of the flow field plates and then subsequently to be fed to the active side of the plates through the backfeed ports. A reactant backfeed port is thus fluidly connected to a manifold port for that reactant via some suitable passage formed in the coolant surface of the plate. And the reactant backfeed port is also fluidly connected to the reactant flow field on the reactant surface of the plate via the passageways of the associated transition region.
US20080113254 for instance discloses exemplary flow field plate constructions incorporating backfeed features. Therein, a disclosed flow field plate assembly comprised first and second flow field plates and a body comprising a porous medium interposed between the first and second flow field plates, the porous medium being operable to allow passage of a fuel and an oxygen-containing gas therethrough, and block from passage therethrough, a flow of liquids to prevent water collection and ice formation, which may block passages formed on at least a portion of the first and/or second flow field plates.
In fuel cell stacks subject to freezing temperatures, accumulations of liquid water can be problematic because, when the water freezes, the ice formed can undesirably block fluid flows or the associated expansion of the solid ice can cause damage to the cell. Significant sized accumulations of liquid water which may be subject to freezing are therefore generally avoided, either by preventing accumulation in the first place or alternatively by removing them before they have the opportunity to freeze. For example, the aforementioned US20080113254 for instance attempts to prevent undesirable water accumulation. Alternatively, various techniques are disclosed in the art for removing water from a fuel cell stack prior to shutdown and storage in subzero temperatures.
However, with the ever changing cell designs, operating conditions, and other advances in the field, problematic accumulations of liquid water may occur in locations and/or under certain conditions where hitherto there was no concern. This invention addresses such issues and provides further related advantages.